Terephthalic acid purge filtration rate by controlling % water in filter feed slurry

ABSTRACT

The process relates improving terephthalic acid purge filtration rate by controlling % water in filter feed slurry and to the recovery of a metal catalyst from an oxidizer purge stream produced in the synthesis of carboxylic acid, typically terephthalic acid, while utilizing pressure filtration.

FIELD OF INVENTION

This invention relates improving terephthalic acid purge filtration rate by controlling weight percent water in filter feed slurry and to the recovery of a metal catalyst from an oxidizer purge stream produced in the synthesis of carboxylic acid, typically terephthalic acid, while utilizing pressure filtration.

BACKGROUND OF THE INVENTION

Terephthalic acid is commercially produced by oxidation of paraxylene in the presence of a catalyst, such as, for example, Co, Mn, Br and a solvent. Terephthalic acid used in the production of polyester fibers, films, and resins must be further treated to remove impurities formed as a result of the oxidation of paraxylene.

Terephthalic acid (TPA) is an intermediate in the production of polyesters for plastics and fiber applications. Commercial processes for the manufacture of TPA are often based on the heavy-metal catalyzed oxidation of p-xylene, generally with a bromide promoter in an acetic acid solvent. Due to the limited solubility of TPA in acetic acid under practical oxidation conditions, a slurry of TPA crystals is usually formed in the oxidation reactor. Typically, the TPA oxidizer slurry is withdrawn from the reactor and TPA solids are separated from the oxidizer mother liquor using conventional solid-liquid separation techniques. The oxidizer mother liquor, which contains most of the catalyst and promoter used in the process, is recycled to the oxidation reactor. Aside from the catalyst and promoter, the oxidizer mother liquor stream also contains dissolved TPA and many by-products and impurities. These by-products and impurities arise partially from minor impurities present in the p-xylene feed stream. Other impurities arise due to the incomplete oxidation of p-xylene resulting in partially oxidized products. Still other by-products result from competing side reactions formed as a result of the oxidation of p-xylene to terephthalic acid. Patents disclosing the production of terephthalic acid such as U.S. Pat. No. 4,158,738 and U.S. Pat. No. 3,996,271 are hereby incorporated by reference in their entirety to the extent that they do not contradict statements herein.

The TPA solids undergo a solid-liquid separation wherein fresh solvent is utilitized to displace a major portion of the liquid component of the oxidizer mother liquor.

Many of the impurities in the oxidizer mother liquor stream that are recycled are relatively inert to further oxidation. Such impurities include, for example, isophthalic acid, phthalic acid and trimellitic acid. Impurities, which may undergo further oxidation are also present, such as, for example, 4-carboxybenzaldehyde, p-toluic acid and p-tolualdehyde. Oxidation inert impurities tend to accumulate in the oxidizer mother liquor upon recycle. The concentration of these inert impurities will increase in the oxidizer mother liquor until an equilibrium is reached whereby the rate of removal of each impurity via the TPA product balances with the rate of formation and the rate of addition to the oxidation process. The normal level of impurities in commercial crude TPA makes it unsuitable for direct use in most polymer applications.

Conventionally, crude TPA has been purified either by conversion a dimethyl ester or by dissolution in water with subsequent hydrogenation over standard hydrogenation catalysts. More recently, secondary oxidative treatments have been used to produce polymer-grade TPA. It is desirable to minimize the concentration of impurities in the mother liquor and thereby facilitate subsequent purification of TPA. In some cases, it is not possible to produce a purified, polymer-grade TPA unless some means for removing impurities from the oxidizer mother liquor stream is utilized.

One technique for impurity removal from a recycle stream commonly used in the chemical processing industry is to draw out or “purge” some portion of the recycle stream. Typically, the purge stream is simply disposed of or, if economically justified, subjected to various treatments to remove undesired impurities while recovering valuable components. One example is U.S. Pat. No. 4,939,297 herein incorporated by reference in their entirety to the extent that they do not contradict statements herein. The amount of purge required for control of impurities is process-dependent; however, a purge amount equal to 10-40% of the total oxidizer mother liquor stream is usually sufficient to produce TPA adequate as feedstock for commercial polymer manufacture. In the production of TPA, the percentage purge of the oxidizer mother liquor stream purge necessary to maintain acceptable impurity concentrations, coupled with the economic value of the metal catalyst and solvent components in the oxidizer purge stream, make simple disposal of the oxidizer purge stream economically unattractive. Thus, there is a need for a process that recovers essentially all of the valuable metal catalysts and acetic acid contained in the oxidizer purge stream while removing a major portion of the impurities present in the oxidizer purge stream. The metal catalyst can be recovered in an active form suitable for reuse by direct recycling to the p-xylene oxidation step.

A number patents teach a terephthalic acid process comprising a purge process comprising concentration, filtration, followed by extraction.

Evaporative concentration of a purge feed comprising acetic acid and water results in a super concentrated purge slurry with lower % weight water content relative to the purge fed because water has a lower boiling point that acetic acid. For example, a purge slurry feed comprising about 94% acetic acid and about 6% water will comprise only about 2.5% water after boiling away about 92% of the purge slurry feed mass.

In has been discovered that the filtration rate of said super concentrated purge slurry in a terephthalic acid purge process can vary greatly depending upon the % water in said super concentrated purge slurry. Variability in the super concentrated purge slurry filtration rate is an attribute of the slurry and is related to the particle size distribution of the solids in the slurry. In this disclosure there are embodiments of the invention that disclosure methods for controlling the % water in the super concentrated purge slurry that improves the filtration rate of said super concentrated purge slurry.

SUMMARY OF THE INVENTION

This invention relates to removal of impurities and the recovery of a metal catalyst from an oxidizer purge stream produced in the synthesis of carboxylic acids, typically terephthalic acid, the process comprising:

-   -   (a) subjecting an oxidizer purge stream formed in a terephthalic         acid process comprising a carboxylic acid, a metal catalyst,         impurities, water and a solvent comprising acetic acid to         evaporation in a 1st evaporator zone to produce a 1^(st) vapor         stream and a concentrated purge stream; and     -   (b) Adding water to said concentrated purge stream in a mixing         zone to produce a water rich concentrated purge stream;     -   (c) subjecting said water rich concentrated purge slurry to         evaporation in a second evaporator zone to produce a 2^(nd)         solvent rich stream and a super concentrated purge slurry         wherein said second evaporator zone comprises an evaporator         operated at a temperature of about 20° C. to about 70° C.;         wherein said super contracted purge slurry has a water content         of about 5 wt % to about 25%     -   (d) filtering said super concentrated purge slurry in a         solid-liquid separation zone to form a filter cake and mother         liquor     -   (e) washing said filter cake with a wash feed in said         solid-liquid separation zone to form a washed filter cake, and         wash filtrate; wherein at least 80% of said metal catalyst from         said super concentrated purge slurry is recovered through said         separation zone into said mother liquor and said wash liquor         cumulative;     -   (f) optionally dewatering said washed filter cake in said         solid-liquid separation zone to form a dewatered filter cake.     -   (g) combining of water with a mother liquor to recover the metal         catalyst and then subjecting an aqueous mixture so formed to         extraction with an extraction solvent to produce an extract         stream and a raffinate stream comprising a metal catalyst.

These embodiments and other embodiments and other embodiments will become more apparent to others with ordinary skill in the art after reading this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment of the invention wherein a method for removing impurities from an oxidizer purge stream formed in a terephthalic acid process comprising acetic acid, water, a metal catalyst, and oxidation impurities by (a) evaporating a portion of an oxidizer purge stream in a first evaporator zone to produce a concentrated purge stream and a 1^(st) vapor stream comprising acetic acid and water (b) adding water in a controlled fashion to the concentrated purge stream in a 1^(st) mixing zone to produce a water rich concentrated purge stream (c) evaporating a portion of water rich concentrated purge stream in a 2^(nd) evaporator zone to form a super concentrated purge slurry stream with a water content ranging from 5.8% to 24.4% and a 2^(nd) vapor stream comprising acetic acid and water; (d) separating solids from the super concentrated purge slurry and subjecting the solids to a wash feed in a solid-liquid separation zone to form a mother liquor stream, a wash liquor stream, and a washed filter cake stream; (e) mixing in a 2^(nd) mixing zone water with the mother liquor stream and optionally a portion of the wash liquor stream to form an aqueous mixture; and (f) adding an extraction solvent to the aqueous mixture in an extraction zone to form an extract stream and a raffinate stream; and (g) feeding the extract stream to a distillation column to form a extraction solvent recycle stream and a sludge stream.

FIG. 2 illustrates different embodiments of the invention wherein the filtration rate of the super concentrated purge slurry varies greatly depending on % water in the stream.

DESCRIPTION OF THE INVENTION

In this specification and in the claims, which follow, reference will be made to a number of terms which shall be defined to have the following meanings:

As used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to a pipe reactor includes one or more pipe reactors.

Ranges may be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another embodiment. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.

“Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where said event or circumstance occurs and instances where it does not. For example, the phrase “optionally heated” means that the material may or may not be heated and that such phrase includes both heated and unheated processes.

Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements.

As used in the specification and the appended claims, the use of “percent” or “%” refers to a weight percent.

In one embodiment of this invention, a process to remove oxidation by-product impurities from an oxidizer purge stream 101 is provided as shown in FIG. 1. The process comprises the following steps.

Step (a) comprises subjecting an oxidizer purge stream 101 to evaporation in a first evaporator zone 120 to produce a 1^(st) vapor stream 121 and a concentrated purge stream 122.

The oxidizer purge stream 101 is withdrawn from a carboxylic acid oxidative synthesis process comprising terephthalic acid. One method for generating oxidizer purge stream 101 is to filter terephthalic acid oxidizer slurry and collect a portion of the mother liquor exiting the filter and routing it to the purge process. Yet another method for generating oxidizer purge stream 101 is to conduct a solvent swap on terephthalic acid oxidation slurry displacing a portion of the oxidation mother liquor and routing it to the purge process. The oxidation mother liquor obtained from a terephthalic acid process can be cooled to a temperature ranging from 90° C. to 45° C. and routed it to a clarification pressure filter such as a candle filter to remove any solids present before routing it to the 1^(st) evaporator in the purge process.

The oxidizer purge stream 101 serves as the feed stream to the present terephthalic acid purge process. The oxidizer purge stream 101 comprises carboxylic acid, water, a solvent, the metal catalyst and impurities. The impurities comprise organic bromides, corrosion metals, p-xylene oxidation by-products, and impurities derived as a result of impurities in the p-xylene. The organic bromides may be used as promoters in the oxidation reaction. Examples of corrosion metals are iron and chromium compounds, which inhibit, reduce or entirely destroy the activity of the metal catalyst. Aside from the catalyst and promoter, the oxidizer mother liquor stream also contains by-products and impurities. These by-products and impurities arise partially from minor impurities present in the p-xylene feed stream. Other impurities arise due to the incomplete oxidation of p-xylene resulting in partially oxidized products. Still other by-products result from competing side reactions in the oxidation of p-xylene to terephthalic acid.

Carboxylic acids include aromatic carboxylic acids produced via controlled oxidation of an organic substrate. Such aromatic carboxylic acids include compounds with at least one carboxylic acid group attached to a carbon atom that is part of an aromatic ring, preferably having at least 6 carbon atoms, even more preferably having only carbon atoms. Suitable examples of such aromatic rings include, but are not limited to, benzene, biphenyl, terphenyl, naphthalene, and other carbon-based fused aromatic rings. Examples of suitable carboxylic acids include, but are not limited to, terephthalic acid, benzoic acid, p-toluic acid, isophthalic acid, trimellitic acid, naphthalene dicarboxylic acid, 2,5-diphenyl-terephthalic acid and mixtures thereof.

Suitable solvents include, but are not limited to, aliphatic mono-carboxylic acids, preferably containing 2 to 6 carbon atoms, or benzoic acid and mixtures thereof and mixtures of these compounds with water. Preferably the solvent is acetic acid mixed with water, in a ratio of about 5:1 to about 25:1, preferably between about 8:1 and about 20:1, and most preferably between about 11:1 and 20:1. Throughout the specification, acetic acid will be referred to as the solvent. However, it should be appreciated that other suitable solvents, such as those disclosed previously, may also be utilized. In the first step of the present process, the oxidizer purge stream 101 is concentrated by conventional means in a first evaporator zone 120 comprising an evaporator to produce a 1^(st) vapor stream 121 and concentrated purge stream 122. The evaporator is operated at atmospheric or slightly super atmospheric conditions, generally from about 1 atmosphere to about 10 atmospheres. The vapor stream 121 comprises a majority of the water and solvent, and the concentrated purge stream 122 comprises the remainder of the water and solvent not removed from the oxidizer purge stream 101. The evaporation removes about 50 wt % to about 85 wt % of the solvent mass present in the oxidizer purge stream 101.

Step (b) subjecting the concentrated purge stream 122 and stream 131 comprising water to mix zone 130 to produce a water rich concentrated purge stream 132. The water rich concentrated purge stream 132 is enriched in water in that the % water in the water rich concentrated purge stream 132 is greater the concentrated purge stream 122. Any equipment know in the art for mixing two liquid streams can be utilized including mixing the two streams in a pipe equipped with an internal static mixer. In an embodiment of the invention, the feed rate (mass/time) of stream 131 added in mix zone 130 is manipulated to control the weight percent (wt %) water contained in the downstream super concentrated purge slurry stream 142 from about 5.0 wt % to about 25.0 wt % water. Another range to control the water content of stream 142 is from about 8.0 wt % water to about 23.0 t % water. Yet another range to control the water content of stream 142 is from about 11.0 wt % water to about 21.0 wt % water. The conduit between the 1^(st) evaporator 120 and mix zone 130 should be maintained at a temperature at or above 90° C. to prevent solids from coming out of solution in the conduit.

Step (c) subjecting said water rich concentrated purge stream 132 to evaporation in a second evaporator zone 140 to produce a 2^(nd) vapor stream 141 and said super concentrated purge slurry stream 142. The conduit between mix zone 130 and the second evaporator 140 should be maintained at a temperature at or above 80° C. to minimize the amount of solids coming out of solution in the conduit.

The second evaporator zone 140 comprises at least one evaporator operated at vacuum conditions. The evaporation can be conducted at a temperature from about 20° C. to about 70° C.; another range is from about 30° C. to about 60° C. The combination of evaporators 120 and 140 are operated so as to concentrate the oxidizer purge stream 101 to a condition wherein about 75 wt % to about 97 wt % of mass of stream 101 is removed by evaporation. Another range for operation of the combination of evaporators 120 and 140 to concentrate the oxidizer purge stream as represented by stream 101 to a condition wherein about 85 wt % to about 94 wt % of the mass of stream 101 is removed by evaporation. Yet, another range for operation of the combination of evaporators 120 and 140 to concentrate the oxidizer purge stream as represented by stream 101 to a condition wherein about 87 wt % to about 93 wt % of the mass of stream 101 is removed by evaporation.

In an embodiment of the present invention, the condition of the super concentrated purge slurry 142 can be as a solid-liquid mixture with only enough solvent to provide pump ability.

Step (d) comprises filtering a super concentrated purge slurry 142 in a filtration zone 160 to form a filter cake 164 and a mother liquor 163; and

Step (e) washing said filter cake 164 with a wash feed 166 in a wash zone 161 to form a washed cake 165 and a wash liquor 167; and optionally dewatering said washed cake 165 in an optional dewatering zone 162 with a gas feed 168 to form a dewatered cake 169. In an embodiment of the present invention, the wash stream 166 comprises water.

In an embodiment of the invention the filtration zone 160 comprises at least one solid liquid separation device. In another embodiment of the invention, the filtration zone 160 and the wash zone 161 and optionally the dewater zone 162 can be accomplished in one solid liquid separation device or in multiple devices in a solid liquid separation zone 200. Example of such devices include but are not limited to continuous pressure filters, continuous vacuum filters, batch pressure filters, centrifuges, and like devices. In another embodiment of the invention, the solid liquid separation zone and the wash zone and the optional dewatering zone can be accomplished in one device. Example of such devices include but are not limited to continuous pressure filters, continuous vacuum filters, batch pressure filters, centrifuges, and like devices

A suitable pressure filter which can be adapted to the requirements of the instant invented process is a BHS-FEST™, rotary drum pressure filter, although other pressure filters which can accomplish the required operation can be used. Examples of other devices that can used in the filtration zone include 160, but are not limited to; vacuum belt filters, filter presses, centrifuges, pressure leaf filters, pressure drum filters, and vacuum drum filters. The pressure filter can be operated at temperature and pressure sufficient to obtain at least 80% recovery of the metal catalyst from the solute of the mother liquor 163. Preferably the pressure filter can be operated at a temperature of about 25° C. to about 80° C., and a pressure of 2 bar to 6 bar gauge.

Step (f) comprises mixing in 2^(nd) mixing zone 170 a water stream 171 with mother liquor stream 163 and optionally a portion of the wash liquor stream 167 to form an aqueous mixture 172. In one embodiment of the invention, the mixing zone 170 comprises a conventional mixer. If necessary, the water 171 can be added to the mixing zone 170 in sufficient quantity to dissolve the metal catalyst in the aqueous mixture stream 172.

The water stream 171 is added to in mixing zone 170 in sufficient quantity to dissolve catalyst resulting in an aqueous mixture 172 where in the ratio of acetic acid to water ranges from about 0.7:1 to 1.4:1, preferably from about 0.8:1 to 1.3:1, and most preferably from about 0.9:1 to 1.2:1. It is desirable to keep the aqueous mixture 172 circulating with an external circulation loop. A small amount of extraction solvent 181, generally about 1 to about 10% by weight, preferably less than 5% by weight may be added to the mixing zone 170 to enhance slurry handling by reducing adherence of solids to the side of vessels. This is represented by the dashed arrow from stream 181 in FIG. 1. It is desirable, but not necessary, to subject the aqueous mixture 172, prior to extraction, to a heat treatment at about 60° C. to about 95° C., another range is about 80° C. about 90° C. for about 0.5 to about 4 hours, preferably about 1 to about 2 hours. By this treatment, organic bromides are reacted to yield inorganic bromides which are preferentially retained in the raffinate stream 183. The quantity of bromine-containing compounds purged from the system along with the unwanted impurities is thereby minimized. The heat treatment conserves bromides and simplifies disposal of the organic impurities.

Step (g) comprises contacting an extraction solvent 181 with the aqueous mixture 172 in an extraction zone 180 to form an extract stream 182 and the raffinate stream 183.

The aqueous mixture 172 is fed to an extraction zone 180 wherein the aqueous mixture 172 and the extraction solvent 181 are contacted in the extraction zone 180. The aqueous mixture 172 and the extraction solvent 181 are mixed to form an extract stream 182 comprising solvent, water, organic impurities, and extraction solvent which forms a lighter phase, and the raffinate stream 183 comprising a metal catalyst, corrosion metals, and water. The extract stream 182 is withdrawn as an overhead stream and the raffinate stream 183 is withdrawn from the bottom of extractor in extraction zone 180. In this invention, one embodiment of the extraction zone 180 is a single stage extractor.

The extraction solvent 181 used in the extractor should be substantially water-immiscible to minimize the amount of organic solvent dissolved in the aqueous fraction. Additionally, the extraction solvent 181 is preferably an azeotropic agent which serves to assist solvent recovery from the organic extract. Solvents which have proven to be particularly useful are C1 to C6 alkyl acetates, particularly n-propyl acetate, isopropyl acetate, isobutyl acetate, sec-butyl acetate, ethyl acetate and n-butyl acetate, although other substantially water-inmiscible organic solvents having an appropriate density and a sufficiently low boiling point may also be used, such as p-xylene. N-propyl acetate and isopropyl acetate are particularly preferred due to their relatively low water miscibility and excellent azeotropic behavior.

The extraction can be effected using solvent ratios from about 1-4 parts by weight extraction solvent per part aqueous mixture. Although the extraction can be operated at ambient temperature and pressure, heating the solvent and extractor to about 30° C. to about 70° C., another range of about 40° C. to about 60° C. can be used. Although the extract stream 109 comprises small amounts of the metal catalyst and corrosion metals, essentially all of the metal catalyst and the majority of the remaining corrosion metals are contained in the heavier phase, raffinate stream 183.

Step (h) comprises separating the extract stream 182 in a separation zone 190 to form a high boiling point organic impurities stream 192 and a recovered extraction solvent stream 191.

The extract stream 182 comprises organic solvent and organic impurities. The extract stream 182 can further comprises acetic acid and water, often in minor amounts. The extract stream 182 may be distilled in a separation zone 190 comprising conventional distillation equipment. Convention distillation equipment includes, for example, a distillation column.

Most of the organic impurities are extracted by the organic solvent in the extraction zone, 180. This occurs because the organic impurities show a high degree of solubility for the organic solvent and to a lesser extent for acetic acid. By distilling the lighter phase from the extractor, the organic solvent is evaporated allowing the organic impurities to concentrate in the column underflow.

The recovered extraction solvent stream 191 may be recycled to the extractor in the extraction zone 180. The high-boiling organic impurities stream 192 are removed as sludge from the base of the distillation column for disposal.

In an embodiment of the invention evaporation zone 120 and 140 are operated in a continuous fashion as opposed to batch operation. In an embodiment of this invention, the purge process is operated in a continuous fashion as opposed to batch operation.

Examples

This invention can be further illustrated by the following examples of other embodiments thereof, although it will be understood that these examples are included merely for purposes of illustration and are not intended to limit the scope of the invention unless otherwise specifically indicated.

The data for Examples 1 through 9 is outlined in Table 1 and FIG. 2 was generated in a laboratory. An objective of these examples is to illustrate the relationship between the % water in the super concentrated purge slurry stream 142 and the filtration rate of the super concentrated purge slurry stream 142 in a pressure filter. It is also an objective of these examples to illustrate how to generate super concentrated purge slurry stream 142 with a range of water contents. The % water in the super concentrated purge slurry stream 142 ranges from 2.5% to 25% and the filtration rate of the super concentrated purge slurry stream 142 ranges from 30 kg filtrate/minute square meter to 1110 kg filtrate/minute square meter respectively.

For each laboratory experiment, oxidizer purge feed 101 was obtained from a commercial plant comprising about 6% water and 94% acetic acid. 75% of the mass of oxidizer purge feed was removed by evaporation at 120° C. in a 1^(st) evaporator zone resulting in a concentrated purge stream 122 comprising about 4.6% water.

In Experiments 1 and 2, the concentrated purge stream 122 was subjected to additional evaporative concentration at a final temperature of at about 55° C. resulting in a super concentrated purge slurry 142 comprising water of about 2.5% and a total evaporative loss of about 92% of the original oxidizer purge feed. The resulting super concentrated purge slurry was filtered in a lab scale pressure filter operated at 3 bar pressure gauge with filtration area of 20 cm², washed with water, and dewatered with N₂. The filtration rate was calculated by dividing the total liquid mass (mother liquor+wash liquor) by the filtration area of 20 cm² and the sum of the filtration time and wash time. For example, the filtration rate for Experiment 1 is =(507 grams/20 cm²/(345 sec+165 sec)=0.0497 grams/cm² second or 29.8 kg/meter² minute.

In Experiments 3 through 9, 120° C. concentrated purge stream 122 was mixed continuously in a mix zone with varying amounts of ambient temperature water and then subjected to additional evaporative concentration at a final temperature of about 55° C. resulting in super concentrated purge stream 142 comprising water ranging from about 8 weight percent (wt %) to 25 wt %. About 92 wt % of the original mass of the oxidizer purge feed is lost during evaporative concentration. The resulting super concentrated purge stream 142 were filtered in a lab scale pressure filter operated at 3 bar pressure gauge with filtration area of 20 cm², washed with water, and dewatered with N₂.

It is instructive to compare Experiments 1 and 5 to see how the addition of water to the concentrated purge stream 122 has a significant impact on the filtration rate of the respective resulting super concentrated purge stream 142. In both examples, about 92% of the oxidizer purge feed is boiled off. In Example 1, no water is added to the concentrated purge stream 122 resulting in a super concentrated purge stream 142 feed to the filter containing 2.5% water and a filtration rate of about 30 kg/meter² minute. In example 5, sufficient water is added to the concentrated purge stream 122 such that the % water in the downstream super concentrated purge stream 142 fed to the filter contains 14% water, and the filtration rate of this super concentrated purge stream 142 slurry is 1,059 kg/meter² minute. It is clear that by adding water to the concentrated purge stream 122 in Example 5, the filtration rate of the resulting downstream super concentrated purge stream 142 increases from 30 kg/meter² minute to 1,059 kg/meter² minute. This is a 3,530% increase in filtration rate.

The addition of water as taught above is not simply a dilution effect but impacts the crystal growth of the solids during evaporative concentration resulting in fewer small particles in the slurry fed to the filter. It is well known in the art that the presence of small particles, particularly at or below 10 microns has a negative impact on a slurries filtration rate. Water addition to the concentrated purge slurry 122 as taught above has a positive impact on the filtration rate of the downstream super concentrated purge slurry fed to the filter by decreasing the weight % of particles smaller than 10 microns in said super concentrated purge slurry. If water is added to the super concentrated purge slurry 142 just prior to filtration, no material improvement in filtration rate is realized. The addition of water must occur at a point in the purge process prior to the final evaporative concentration zone. For example, the water addition step can occur after the 1^(st) evaporative concentration zone in a purge process with two evaporative concentration zones. The water addition can also occur prior to the 1^(st) evaporator zone but is not desirable due to the higher energy costs associated with evaporative concentration prior to the filtration zone 200.

It is clear from the data plotted in FIG. 2 below that it is desirable to control the % water contained in the super concentrated purge slurry stream feed 142 to the filter from 5.8% to 24.4% water. A more preferable range to control the water content of stream 142 is from 7.9% water to 22.8% water. A still more preferable range to control the water content of stream 142 is from 10.5% water to 20.4% water. The most preferred range to control the water content of stream 142 is from 12.5% water to 18.9% water. It is also clear that the physical location in the purge process where water is added to achieve this improvement in stream 142 filtration rate will be located prior to the last evaporation zone in the purge process.

TABLE 1 Experimental Filtration Data for Super Concentrated Purge Slurry (SCPS) Total Liquid Flux Data % Water Filtration Zone Data Wash Zone Data Mother Liquor Combined in SCPS SCPS Mother Wash With Wash Filtrate Feed to Feed Filt. Liquor Wash Wash Filtrate kG Expt. Filter Mass Time Mass Mass Time Mass Mass grams min. # (% W) (g) (sec) (g) (g) (sec) (g) (g) sec. M² 1 2.5%  400.3 345.0 350.0 150 165 157.7 507.7 1.00 30 2 2.5%  400.5 30.0 103.4 180 654 429.9 533.3 0.78 23 3 7.7%  400.8 16.0 306.4 76.00 9.1 132.6 439.0 17.49 525 4 11% 400.2 13.0 321.8 98.03 6.1 150.4 472.2 24.75 742 5 14% 400.3 8.3 309.3 98.00 5.2 167.4 476.7 35.31 1059 6 14% 400.1 8.6 303.1 98.00 5.7 173.4 476.5 33.32 1000 7 18% 400.2 8.0 313.1 140 5.9 201.4 514.5 37.01 1110 8 20% 400.4 18.3 351.2 65.00 5.2 101.5 452.7 19.26 578 9 25% 400.0 27.0 323.7 116.7 13.6 168.2 491.9 12.12 363 

We claim:
 1. A terephthalic acid purge process comprising: (a) subjecting an oxidizer purge stream formed in a terephthalic acid synthesis process comprising a carboxylic acid, a metal catalyst, impurities, water and a solvent comprising acetic acid to evaporation in a 1^(st) evaporator zone to produce a vapor stream and a concentrated purge slurry; and (b) adding water to said concentrated purge slurry in a 1^(st) mix zone to produce a water rich concentrated purge slurry; (c) subjecting said water rich concentrated purge slurry to evaporation in a 2^(nd) evaporator zone to produce a solvent rich stream and a super concentrated purge slurry wherein said second evaporator zone comprises an evaporator operated at a temperature of about 20° C. to about 70° C.; wherein said super concentrated purge slurry has a water content of about 5 wt % to about 25 wt % (d) filtering said super concentrated purge slurry in a solid-liquid separation zone to form a filter cake and mother liquor (e) washing said filter cake with a wash feed in said solid-liquid separation zone to form a washed filter cake and a wash liquor.
 2. The process according to claim 1 wherein at least 80% of said metal catalyst from said super concentrated purge slurry is recovered through said separation zone into said mother liquor and said wash liquor cumulative.
 3. The process according to claim 2 wherein about 75 wt % to about 95 wt % of said solvent and water is removed by evaporation from said oxidizer purge stream in step (a) and step (c) combined.
 4. The process according to claim 3 where in at least a portion of said washed filter cake is routed to a stream comprising terephthalic acid downstream of oxidation zone.
 5. The process according to claim 1 wherein said super concentrated purge slurry has a water content of about 8 wt % to about 23%.
 6. The process according to claim 1 wherein said super concentrated purge slurry has a water content of about 11 wt % to about 21%.
 7. The process according to claim 1 wherein said super concentrated purge slurry has a water content of about 13 wt % to about 18%.
 8. The process according to claim 1 wherein the temperature of said concentrated purge stream routed from said 1^(st) evaporation zone to said mix zone is maintained above 80 C.
 9. The process according to claim 1 wherein the residence time in said second evaporation zone ranges from 30 minutes to 180 minutes.
 10. The process according to claim 1 wherein the filtration rate of said super concentrated purge slurry in said solid liquid separation zone is higher for water content range of said super concentrated purge slurry ranging from 13% to 19% than in any water concentration outside said range.
 11. A terephthalic acid purge process comprising: (a) subjecting an oxidizer purge stream formed in a terephthalic acid synthesis process comprising a carboxylic acid, a metal catalyst, impurities, water and a solvent comprising acetic acid to evaporation in a 1^(st) evaporator zone to produce a vapor stream and a concentrated purge slurry; and (b) adding water to said concentrated purge slurry in a 1^(st) mix zone to produce a water rich concentrated purge slurry; (c) subjecting said water rich concentrated purge slurry to evaporation in a 2^(nd) evaporator zone to produce a solvent rich stream and a super concentrated purge slurry wherein said second evaporator zone comprises an evaporator operated at a temperature of about 20° C. to about 70° C.; (d) filtering said super concentrated purge slurry in a solid-liquid separation zone to form a filter cake and mother liquor (e) washing said filter cake with a wash feed in said solid-liquid separation zone to form a washed filter cake and a wash liquor; wherein the mass of water added in the mix zone in step (b) is manipulated to result in the super concentrated purge slurry of step (c) having a water content of from 12.5% to 18.9% water.
 12. The process according to claim 11, wherein at least 80% of said metal catalyst from said super concentrated purge slurry is recovered through said separation zone into said mother liquor and said wash liquor cumulative.
 13. The process according to claim 12, wherein about 75 wt % to about 95 wt % of said solvent and water is removed by evaporation from said oxidizer purge stream in step (a) and step (c) combined.
 14. The process according to claim 13, wherein at least a portion of said washed filter cake is routed to a stream comprising terephthalic acid downstream of oxidation zone.
 15. The process according to claim 11, wherein the temperature of said concentrated purge stream routed from said 1⁵ evaporation zone to said mix zone is maintained above 80 C.
 16. The process according to claim 11, wherein the residence time in said second evaporation zone ranges from 30 minutes to 180 minutes.
 17. The process according to claim 11, wherein the filtration rate of said super concentrated purge slurry in said solid liquid separation zone is higher for water content range of said super concentrated purge slurry ranging from 13% to 19% than in any water concentration outside said range. 